Sulphite process



y 1943. G. v. PALMROSE EI'IAL 2,320,294

SULPHITE PROCESS Filed April 25', 1940 n a Q mm a A N N R O w w M A .30m

Patented May 25, 1943 UNITED STATES PATENT OFFICE corporation oiWashington Application April 23, 1940, Serial No. 331.084

v2 Claims.

This invention relates to the acid sulphite pulping process andparticularly to improvements relating to the economy of such operationsand the recovery of material from the waste liquor which may bereutilized in the process.

In a co-pending application Serial No. 326,843,

filed March 30, 1940, it is shown that the Norwegian Patent No. 40,517issued December 8, 1024, fails to disclose a practical and commercialoperation. This patent suggests the use of magnesium sulphite as acooking liquor in the sulphite process to produce a waste sulphiteliquor which may be evaporated to dryness and burned. The resultingmagnesium oxide may then be combined with sulphur dioxide from thecombustion gases to produce magnesium sulphite liquor. The disclosure isdefective in that it fails to take account of the necessity forproviding a large proportion of free sulphur dioxide in the liquor usedas a cooking medium, and no provision is made for the introduction offree sulphur dioxide to the magnesium sulphite liquor. The process asdescribed in the Norwegian patent has never been practised commercially,and so far as is known the problems which arise in attempting to applythe procedure have not been considered or solved in a practicablemanner.

The application aforesaid emphasizes the necessity for providing freesulphur dioxide in the cooking liquor. While the free sulphur dioxidedoes not enter directly into the reaction, it nevertheless exerts adriving force which speeds up the hydrolysis and sulphonation of thelignin and accomplishes the liberation of the cellulose much morerapidly and at lower temperatures than is otherwise possible. Thatapplication also indicates that the necessary free sulphur dioxide canbe supplied to the cooking liquor by absorbing the sulphur dioxiderelieved from the digester during and after the cooking operation, andthat losses incident to the operation may be made up by the introductionof sulphur dioxide supplied by a sulphur burner, the gases from whichnormally carry approximately 14-18% of sulphur dioxide.

It is the object of the present invention to ing liquor, including theprovision of a liquor of the necessary strength in respect to combinedand free sulphur dioxide, and permitting the recovery of sulphur dioxidewhich otherwise would be lost in the evaporation of the waste liquorprior'to burning thereof.

Other objects and advantages of the invention will be apparent as it isbetter understood by aflordfurther improvements in the sulphite processbased upon the theory of the Norwegian patent and adapted to permit thecommercial practice of the procedure and the attainment of advantagesresulting from its practical application.

Another object of the invention is the provision of a sulphite processin which waste products are consumed to permit regeneration of thecookreference to the following speciiicationand the accompanyingdrawing, which illustrates diagrammatically apparatus suitable lor theapplication of the process in commercial operations. Details of thestructure are eliminated for the purpose'of clarity.

when limo-cellulose material is heated with an acid sulphite liquor, thereaction is one of hydrolysis, in which the bond between the lignin andthe hemi-celluloses or cellulose is broken. This reaction is followedalmost immediately by sulphonation of the lignin to form ligno-sulphonicacid which, if allowed to exist in the free state, results in thehydrolysis of the ligno-cellulose compounds too rapidly, with the resultthat the lignin polymerizes and is no longer capable of forming solublesulphonic acid. In the presence of a base such as calcium, magnesium orsodium, present as a bisulphite compound, the metal ion combines withthe ligno-sulphonic acid as fast as it is formed to produce a metal saltof the sulphonic acid. The reaction involving hydroly'sis andsulphonation will then proceed smoothly, and a cellulose relatively freefrom polymerized lignin will be produced.

Experience has shown that in relation to the charge of wood on a bonedry basis, a definite minimum ratio of sulphur dioxide combined with themetal ion must exist'in order to secure satisfactory pulping. This ratlofor coniferous woods has been found to lie between .04 and .055 poundsof sulphur dioxide combined with the metal per pound of wood on the bonedry basis. While it is possible to cook wood and to liberate thecellulose therefrom with a metal bisulphite of such concentration thatthe sulphur dionde is combined with the metal in the ratio mentioned,the cooking operation is relatively slow, and high temperatures arenecessary. Both of the latter conditions are undesirable in a commercialoperation, because of the cost and for other reasons.

To permit rapid cooking at lower temperatures, it is necessary, asalready indicated, to provide free or uncombined sulphur dioxide in thecooking liquor. All of this free sulphur dioxide which is essential tothe practical commercial operation of the sulphite process, iseventually relieved from gester of the usual type.

the digester, either during the cooking operation or at the terminationthereof. It is necessary to provide, therefore, for the constantcirculation of a large quantity of free sulphur dioxide between thedigester and the fresh cooking liquor which is introduced thereto. Wasteof sulphur dioxide must be avoided, so far as'is practicable.

There is inevitably some loss of free sulphur dioxide in operationsconducted under the best possible conditions. We have discovered that aparticular source of such losses in the process contemplated resultsfrom the partial decomposltion of organic sulphur compounds when theacid waste liquor is evaporated. This waste liquor, at the conclusion ofthe cooking operation, contains substantially no free sulphur dioxideifthe digester pressure has been reduced to normal atmospheric pressurebefore the liquor and pulp are removed from the digester. However, whenthe waste liquor is. subjected to vacuum evaporation in order to producea concentrated waste liquor which may be burned to recover sulphurdioxide, the base and heat, there results in the evaporator a partialdecomposition of the organic sulphur compounds, and we have found thatthe amount of sulphur dioxide relieved in the course of such evaporationamounts to 0.3 to 0.4 pounds of sulphur dioxide per hundred pounds ofwater evaporated. This represents a considerable proportion, from 10 to20%, of the total sulphur recoverable from the waste liquor.

The sulphur dioxide which is freed from the organic compounds duringevaporation may be recovered readily by condensing the vapors from theevaporating system. A suitable heat exchanger, through which the vaporsmay be conducted, cools them sufllciently to condense and remove thebulk of the water therein. leaving sulphur dioxide which may be utilizedadvantag eously to fortify the sulphite liquor, afiording a part of thefree sulphur dioxide which must be necessarily provided therein beforethe liquor is utilized for cooking in the digester. This sulphur dioxiderecovered from evaporation of the waste liquor, together with thesulphur dioxide relieved from the digester during and after the cookingoperation, is sufiicient usually to maintain the cooking liquor at therequired strength. It is possible to make suitable additions, both ofsulphur dioxide and of the base employed, to meet any losses which mayoccur in the system. This is best accomplished by introducing magnesiumsalts and sulphur in the required proportions into the combustionchamber when the evaporated liquor is burned.

The procedure as described solves practicably and in the manner toafford a commercially economical operation, the problems incident to asulphite process in which magnesia is used as a base, so that; the wasteliquor can be burned and the magnesium oxide ash can be recombined withsulphur dioxide recovered in the system to afford a satisfactory cookingliquor. The resulting process is adapted to effect very substantialsavings in the operation of the sulphite process, and particularly toavoid the discharge of undesirable waste products to the atmosphere andto streams which have been polluted heretofore by the discharge of wasteproducts from sulphite plants.

Referring to the drawing, indicates a di The details of such a structureare well known and require no description. The digester is adapted to besupplied at intervals with ligno-cellulose material slum bisulphitecontaining the desired ratio of combined sulphur dioxide and sufllcientfree sul- Dhur dioxide absorbed therein to permit the rapid completionof the cook and the satisfactory recovery of cellulose.

T afford this cooking liquor, we provide storage vessels 8, I, 8 and 8,the first two being pressure vessels. During the cooking operation whichis conducted at pressures usually between 70 and 90 pounds per squareinch, a certain amount of free sulphur dioxide is relieved from thedigester 5 and is conducted by a pipe l0 into the vessel 6 where it isabsorbed in the strong cooking liquor. Any surplus of sulphur dioxidenot absorbed in the first vessel is conducted by a pipe II to the secondvessel 1 which likewise receives sulphur dioxide relieved from thedigester after the conclusion of the cook at pressures from pounds downto 15 pounds per square inch through pipes i2 and I3. The vesel 1contains the weaker liquor which is eventually delivered to the vessel 6through a pump it and pipe l5. Circulation of the liquor in the vessels8 and I is provided by a pipe [6 and pump l1 and by a pipe l8 and pump[9 respectively.

Sulphur dioxide relieved from the digester at pressures from 15 poundsper square inch to 0 is delivered to the'vessel 8 through the pipe II,where it is met by circulating liquor delivered by pump 24 through pipe25. Also, any sulphur dioxide which is not absorbed in the vessel 1passes through the pipe 20 into the vessel 8, from which the weak liquoris delivered by a pump 2| and pipe 22 into the vessel 1. Finally, thevessel 9 affords storage for the weak bisulphite liquor produced ashereinafter described. This liquor is delivered through pipe 23 to thevessel 8 as required. The liquor passes successively through the vessels1 and 6 respectively as hereinbefore indicated, where it absorbs moresulphur dioxide until it reaches the ultimate strength required forcooking liquor and is introduced into the digester 5 through a pipe 26.

Although the digester may be operated as in the customary cookingoperation, it is preferred to use the low liquor ratio as described inthe patent to George V. Palmrose, No. 2,192,239

and with a cooking liquor consisting of magneissued March 5, 1940, andprovision is made, therefore, for circulation of the liquor through apump 21 and pipe 28 to a heat exchanger 29 wherein the liquor may beheated by means of steam or otherwise, and thence through a pipe 30 intothe digester.

At the conclusion of the cooking operation, the free sulphur dioxide isrelieved and delivered as hereinbefore described into vessel 8. Thecontents of the digester, including the pulp and the acid waste liquor,are then delivered by a pump 3i and pipe 32 into a tank 33 from whichany remaining gases, including steam, escape through a pipe 34. The pulpand the waste liquor is delivered by a pump 35 and pipe 36 to a seriesof filter washers 31. For convenience, only the last washer of theseries is illustrated. It is supplied with wash water through the pipe38. The pulp is withdrawn from a launder 39. The waste liquor escapesthrough a pipe 40 to a tank Q It will be understood that in the filterwashers pulp advances counter-current to the travel of the washingliquid, being finally washed with fresh water, the wash liquid travelingthrough the series of washers and combining with the waste liquor whichaccumulates in the tank ll.

The waste liquor is delivered by a pump 42 and pipe 43 to a scrubbingtower 44 through which it is circulated by a pump 45 and pipe 46 whichdelivers the liquid in sprays 41. The liquor is then withdrawn throughthe pump 45 and pipe 48 and delivered to a bank of multiple eflectevaporators 49. The liquor is circulated through the successive elementsof the bank of evaporators by pipes 58 and pumps counter-current to thevapors which travel through pipes 52. A heating agent such as steam isintroduced through a pipe 53, and the vapors escape through a pipe 54 toa condenser 55 where, as hereinbefore indicated. water is condensed fromthe vapors, leaving sulphur dioxide which isdelivered through a pipe 58.The condensed water passes through a pipe 51 to a sump'58. The sulphurdioxide, substantially free from water vapor, passes from the pipe 56into a jet condenser 59 where it is mingled with magnesium bisulphiteliquor produced as hereinafter described and accumulated in the tank 60whence it is circulated through the pump 6|, pipe 62, heat exchanger 63and pipe 64 to the jet condenser 59. Thus, the sulphur dioxide relievedfrom the evaporating system is reco ered as free sulphur dioxideabsorbed in the magnesium sulphite liquor which may be delivered throughthe pump GI and pipe 65 to the vessel 9 where it affords the basis forthe cooking liquor being already fortified in free sulphur dioxide andabsorbing in the vessels 8, I and 6, respectively, additional quantitiesof sulphur dioxide, to produce the strong cooking liquor.

Alternatively, the sulphur dioxide from the condenser 55 may be drawnthrough-a reciprocating vacuum pump 33 which will compressthe gas to therequired pressure to permit delivery thereof through the pipe 94 to oneof the vessels 6 or 1 as may be desired. In that case, the liquor fromthe tank 80 will pass directly to the vessel 9.

The concentrated waste sulphite liquor from the evaporation system isdelivered through a pump 56 and pipe 81 to a furnace 68 where it isburned with air supplied through a pipe 89 and blower l8. Magnesiumcompounds and sulphur may be introduced at this point if needed to makeup losses. The air may be preheated in a heat exchanger H by products ofcombustion delivered from the furnace by the pipe 12. The air issupplied through a pipe 13 and is delivered to the blower by a pipe 14.

Combustion of the liquor in the furnace results in a pulverulent ash ofmagnesium oxide. The heat generated by combustion may be converted intosteam in a boiler 15, and the steam may be withdrawn through a pipe 16and 'utilized for any desired purpose, as for example in the evaporatingsystem or elsewhere in the plant.

The hot combustion gases carrying the bulk of the finely dividedmagnesium oxide ash are delivered through a pipe 11 and blower 18 into adust catcher 19 from which the gases escape and are delivered rough ablower 88 and pipe 8i to the scrubbing tower 44 previously described.The magnesium oxide ash is delivered to a tank 82 and supplied withwater through a pipe 83 whichconverts it into a slurry. This slurry isdelivered through a pipe 84 to a tank 85 whence itis delivered by pump86 and pipe 81 to a packed absorption tower 88. In this tower the slurryis subjected to the cool furnace gases containing sulphur dioxidederived from the combustion of the waste liquor. These gases aredelivered from the scrubbing tower 44 to the packed absorption tower 88by a pipe 89. The sulphur dioxide in the gases is combinbed with themagnesium oxide in the slurry to produce magnesium bisulphite which iswithdrawn through the pipe 88 and delivered to the tank 88. The balanceof the furnace gases escapes through a pipe 9| substantially free fromsulphur dioxide. Circulation of the slurry during theabsorption'operation is permitted by apipe connection 82 to the tank 85.An additional absorption tower, similar to the tower 88, may beutilized, if desired, to facilitate recovery of the sulphur dioxide inthe furnace gases.

As will be seen from the foregoing description, the operation affords acomplete cycle in which strong cooking liquor, fortified with thenecessary amount of free sulphur dioxide, is pro duced continuously foruse in the digester. The sulphur dioxide relieved from the digester andsulphur dioxide recovered during evaporation of the waste liquor and bythe subsequent combustion thereof, areutilized to produce and to fortifythe magnesium bisulphite liquor, affording fresh quantities of theliquor without necessary additions from other sources. There are nowaste products to be discharged. Losses, if any, can be made up by theaddition of sulphur dioxide from other sources and of the base metal asmay be required. In general, however, the free sulphur dioxide operatesin a cycle in which it performs its function in the digester and isrecovered and reutilized in the preparation of fresh cooking liquor.

The magnesia which is the metallic base neces sary in the process issimilarly recovered and travels in a cycle in which it is combined withsulphur dioxide to produce cooking liquor. The organic material, freedfrom sulphur, is burned, and the heat of combustion is utilized toproduce steam employed in the operation. Thus, the efflciency of theprocess is maintained, and uneconomic factors are eliminated, includingthe discharge of waste products to rivers and streams and to theatmosphere.

Various changes may be made in the form and construction and operationof the apparatus, and in the details of procedure, without departingfrom the invention or sacrificing any of the advantages thereof.

We claim:

1. A cyclic method of treating ligno-cellulosic material which comprisesthe essential steps of cooking the ligno-cellulosic material underpressure with a solution of magnesium bisulphite containing asubstantial excess of free sulphur dioxide, releasing gases during andafter the cooking, separating the acid waste liquor from the treatedcellulosic material after completion of the cooking operation,evaporating and burning the separated acid waste liquor to produce anash consisting mainly of magnesium oxide and combustion gases containingsulphur dioxide. adding magnesium compounds and sulphur as required tomake up losses of the evaporated liquor during the burning of the wasteliquor. making a slurry of the magnesium oxide ash. absorbing thesulphur dioxide in said combustion gases in said slurry to reformmagnesium bisulphite to be used in a-sub'sequent cooking operation,condensing the vapor resulting from the evaporation of the waste liquorto separate sulphur dioxide therefrom, absorbing the separated sulphurdioxide in the magnesium bisulphite liquor produced from the ash and thesulphur dioxide of the combustion gases, and returning the reformedmagnesium bisulphite containing the absorbed sulphur dioxide for furthercooking of ligno-cellulosic material.

2. A cyclic method or treating liano-cellulosic material which comprisesthe essential steps of cooking the ligno-cellulosic material underpressure with a solution of magnesium -bisulphite containing asubstantial excess of free sulphur dioxide, releasing gases during andafter the cooking, separating the acid waste liquor from the treatedcellulosic material after completion of the cooking operation,evaporating and buming the separated acid waste liquor to produce an ashconsisting mainly of magnesium oxide and combustion gases containingsulphur dioxide, adding magnesium compounds and sulphur as required tomake up losses of the evaporated liquor during the burning of the wasteliquor, making a slurry of the magnesium oxide ash, absorbing thesulphur dioxide in said combustion gases in said slurry to reform;magnesium bisulphite to be used in a subsequent cooking operation,condensing the vapor resulting from the evaporation of the waste liq toseparate sulphur dioxide therefrom, absorbing the separated sulphurdioxide in the magnesium bisulphite liquor produced from the ash and thesulphur dioxide of the combustion gases, thereafter fortifying thereformed magnesium bisulphite liquor with sulphur dioxide relieved fromthe digester at low pressure and thereafter at higher pressure, andreturning the reformed magnesium bisulphite liquor containing theabsorbed sulphur dioxide for further cooking of ligno-cellulosicmaterial.

GEORGE VICTOR PALMROSE.

DONALD KEITH MACBAIN.

